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The M60E4 (Mk 43 as designated by the US Navy) was pitted against the (then called) M240E4 in Army trials during the 1990s for a new infantry General Purpose Machine Gun, in a competition to replace the decades-old M60s. The M240E4 won, and was then classified as the M240B. This led to 1000 existing M240 being sent to FN for an overhaul and a special kit that modified them for use on ground (such as a stock, a rail, etc.). This led to procurement contracts in the late 1990s for all-new M240B. However, a new feature was added, a hydraulic buffer system to reduce the felt recoil as was incorporated in the earlier M60. While the M240B had been more reliable in the tests, it was a few pounds heavier than the M60E4, and there has been a program underway for a new lighter weight General Purpose Machine Gun since the early 2000s. The Army M240 converted the M240B configuration should not be confused with the large numbers of M240D converted to the M240G configuration for the Marine Corps.

The M240 family of weapons are gas-operated, belt-fed, air-cooled automatic weapons. Infantry M240s (M240B for the US Army and M240G for the US Marine Corps) are fitted with the pistol grip and trigger, and the wooden (early models) or polymer (present manufacture) stock. Coaxial guns (like the M240C) have the trigger replaced by the electric solenoid, and the pintle-mounted versions (like the M240D) have the spade grips instead of the pistol grip and the stock.

The M240 uses the long stroke gas piston system with a gas regulator, located below the barrel. The bolt is locked using a swinging shoulder that engages the cut in the floor of the receiver.

It is fed using disintegrating steel belts of various lengths. The rate of fire can be set between low (650 rpm) and high (950 rpm) by adjusting the gas regulator and return spring assemblies. As is common for a machine gun, the M240 can only be fired in full auto.

The receiver is made from stamped steel, and the charging handle is located on the right side of the receiver. There are two mounting points on the bottom of the receiver to fit into various mountings such as infantry tripods and vehicle or boat pintle mounts.

The M240 typically uses iron sights, with the rear sight adjustable for elevation in 100m range increments while the front sight is drift adjustable for windage. Most production variants have Picatinny-style optic mounts on the top of the receiver.

Use of the weapon requires several distinct steps that depart from other infantry small arms due to the open bolt nature of the weapon. "Half Load" is accomplished by making sure the weapon is clear, then running the bolt forward and placing an ammunition belt on the feed tray and closing the top cover. "Full Load" is then required to make the weapon ready to fire by running the charging handle to the rear and returning it forward (manually for infantry variants, via spring for vehicle variants). The weapon is then either placed on Safe or aimed on target in preparation for firing. Great care must be used to ensure the position of the bolt is positively locked either rearward or forward. If the bolt is unsecured and allowed to run forward with ammunition in the feed tray, the weapon will fire regardless of the Safety setting or the trigger being pulled.

Clearing the weapon is performed by ensuring the bolt is locked to the rear and the weapon is on safe. The feed tray cover is then lifted, the remaining belt (if any) is swept out of the feed tray, the feed tray is lifted to visually inspect the rear of the barrel and the face of the bolt. Any links or brass casings are removed. The weapon is now clear.

In the rare event that a live round is on the bolt face, it is knocked loose with a cleaning rod or another rigid object. If there is a live round lodged in the barrel, the operator must immediately decide if the barrel is hot enough that there is a chance of it cooking off. If there is, he will immediately move his face away from the opening of the weapon. He should then wait for the barrel to cool off before attempting to remove it. In many cases, attempting to remove the barrel will cause the round to detonate as soon as the barrel is unlocked from the receiver. The operator can also attempt to extract the round by taking the weapon off of safe, pulling the trigger and pulling back on the charging handle. This has a fair chance of causing the weapon to fire, so care should be made in ensuring that the weapon is pointed in a safe direction first.

The M240B is the standard infantry general purpose machine gun of the U.S. Army and is the variant in use by the U.S. Air Force, U.S. Navy and U.S. Coast Guard. It comes configured for ground combat. It is almost always referred to as an "M240 Bravo" or even a "240 Bravo" verbally, but always written as M240B.

The M240D, a variant of the M240, has two possible configurations: aircraft and egress (ground). The aircraft configured M240D has a front and rear sight and a trigger group which accommodates the spade grip device. The ground configuration involves the installation of an Egress Package or "infantry modification kit" which is designed to provide downed aircrew personnel with increased fire power. The M240D is an upgrade of the M240E1, primarily in the addition of an optical rail on the receiver cover. The M240E1 had also fitted with spade grips for flexible use.

The M240G (referred to as the "240 Golf"), was the standard U.S. Marine Corps medium machine gun. The Marine Corps has replaced the M60E3 with the M240G. The M240 allows for commonality throughout the Marine Corps whether the weapon is used in an infantry, vehicular, or airborne role. The M240G is the ground version of the original M240 or M240E1, 7.62mm medium class weapon designed as a coaxial/pintle mounted machine gun for Tanks and LAVs. The M240G can be modified for ground use by the installation of an "infantry modification kit," (a flash suppressor, front sight, carrying handle for the barrel, a buttstock, infantry length pistol grip, bipod, and rear sight assembly). The 240G lacks a front heat guard, and is few pounds lighter at 25.6 pounds, than the M240B.

An improvement of the M240D, the M240H (sometimes referred to as the "240 Hotel") features a rail equipped feed cover, an improved flash suppressor, and has been configured so that it can be more quickly converted to infantry standard using an Egress Kit.

The M240L is a lightweight variant of the M240B. It has a shorter barrel (four inches shorter) than the M240B and incorporates titanium construction and other manufacturing methods to produce lighter parts. The M240L is about 5.5 pounds lighter than the M240B.